1. Field of the Invention
The invention pertains to the field of variable camshaft timing VCT) systems. More particularly, the invention pertains to a method to compensate for VCT phaser error over speed range.
2. Description of Related Art
Consideration of information disclosed by the following U.S. Patents, which are all hereby incorporated by reference, is useful when exploring the background of the present invention.
U.S. Pat. No. 5,002,023 describes a VCT system within the field of the invention in which the system hydraulics includes a pair of oppositely acting hydraulic cylinders with appropriate hydraulic flow elements to selectively transfer hydraulic fluid from one of the cylinders to the other, or vice versa, to thereby advance or retard the circumferential position on of a camshaft relative to a crankshaft. The control system utilizes a control valve in which the exhaustion of hydraulic fluid from one or another of the oppositely acting cylinders is permitted by moving a spool within the valve one way or another from its centered or null position. The movement of the spool occurs in response to an increase or decrease in control hydraulic pressure, PC, on one end of the spool and the relationship between the hydraulic force on such end and an oppositely direct mechanical force on the other end which results from a compression spring that acts thereon.
U.S. Pat. No. 5,107,804 describes an alternate type of VCT system within the field of the invention in which the system hydraulics include a vane having lobes within an enclosed housing which replace the oppositely acting cylinders disclosed by the aforementioned U.S. Pat. No. 5,002,023. The vane is oscillatable with respect to the housing, with appropriate hydraulic flow elements to transfer hydraulic fluid within the housing from one side of a lobe to the other, or vice versa, to thereby oscillate the vane with respect to the housing in one direction or the other, an action which is effective to advance or retard the position of the camshaft relative to the crankshaft. The control system of this VCT system is identical to that divulged in U.S. Pat. No. 5,002,023, using the same type of spool valve responding to the same type of forces acting thereon.
U.S. Pat. Nos. 5,172,659 and 5,184,578 both address the problems of the aforementioned types of VCT systems created by the attempt to balance the hydraulic force exerted against one end of the spool and the mechanical force exerted against the other end. The improved control system disclosed in both U.S. Pat. Nos. 5,172,659 and 5,184,578 utilizes hydraulic force on both ends of the spool. The hydraulic force on one end results from the directly applied hydraulic fluid from the engine oil gallery at full hydraulic pressure, PS. The hydraulic force on the other end of the spool results from a hydraulic cylinder or other force multiplier which acts thereon in response to system hydraulic fluid at reduced pressure, PC, from a PWM solenoid. Because the force at each of the opposed ends of the spool is hydraulic in origin, based on the same hydraulic fluid, changes in pressure or viscosity of the hydraulic fluid will be self-negating, and will not affect the centered or null position of the spool.
U.S. Pat. No. 5,289,805 provides an improved VCT method which utilizes a hydraulic PWM spool position control and an advanced control algorithm that yields a prescribed set point tracking behavior with a high degree of robustness.
In U.S. Pat. No. 5,361,735, a camshaft has a vane secured to an end for non-oscillating rotation. The camshaft also carries a timing belt driven pulley which can rotates with the camshaft but which is oscillatable with respect to the camshaft. The vane has opposed lobes which are received in opposed recesses, respectively, of the pulley. The camshaft tends to change in reaction to torque pulses which it experiences during its normal operation and it is permitted to advance or retard by selectively blocking or permitting the flow of engine oil from the recesses by controlling the position of a spool within a valve body of a control valve in response to a signal from an engine control unit. The spool is urged in a given direction by rotary linear motion translating means which is rotated by an electric motor, preferably of the stepper motor type.
U.S. Pat. No. 5,497,738 shows a control system which eliminates the hydraulic force on one end of a spool resulting from directly applied hydraulic fluid from the engine oil gallery at full hydraulic pressure, PS, utilized by previous embodiments of the VCT system. The force on the other end of the vented spool results from an electromechanical actuator, preferably of the variable force solenoid type, which acts directly upon the vented spool in response to an electronic signal issued from an engine control unit (“ECU”) which monitors various engine parameters. The ECU receives signals from sensors corresponding to camshaft and crankshaft positions and utilizes this information to calculate a relative phase angle. A closed-loop feedback system which corrects for any phase angle error is preferably employed. The use of a variable force solenoid solves the problem of sluggish dynamic response. Such a device can be designed to be as fast as the mechanical response of the spool valve, and certainly much faster than the conventional (fully hydraulic) differential pressure control system. The faster response allows the use of increased closed-loop gain, making the system less sensitive to component tolerances and operating environment.
Furthermore, it is known in the art to use negative feedback loop for controlling variable camshaft timing (VCT) systems. U.S. Pat. No. 5,289,805 describes an improved closed loop feedback system for a VCT system. The same patent further teaches a robust control law used in the closed loop feedback system for a VCT system. The control law includes a phase integration (PI) block and a phase lead block. FIGS. 1 and 1A show the feedback loop and the control law respectively.
Referring to FIG. 1, a prior art feedback loop 10 is shown. The control objective of feedback loop 10 is to have the VCT phaser at the correct phase (set point 12) and the phase rate of change is zero. In this state, the spool valve 14 is in its null position and no fluid flows between two fluid holding chambers of a phaser (not shown). A computer program product which utilizes the dynamic state of the VCT mechanism is used to accomplish the above state.
The VCT closed-loop control mechanism is achieved by measuring a camshaft phase shift .θ0 16 and comparing the same to the desired set point 12. The VCT mechanism is in turn adjusted so that the phaser achieves a position which is determined by the set point 12. A control law 18 compares the set point 12 to the phase shift θ0 16. The compared result is used as a reference to issue commands to a solenoid 20 to position the spool 14. This positioning of spool 14 occurs when the phase error (the difference between set point 12 and phase shift θ0 16) is non-zero.
The spool 14 is moved toward a first direction (e.g. right) if the phase error is positive (retard) and to a second direction (e.g. left) if the phase error is negative (advance). When the phase error is zero, the VCT phase equals the set point 12 so the spool 14 is held in the null position such that no fluid flows within the spool valve.
Camshaft and crankshaft measurement pulses in the VCT system are generated by camshaft and crankshaft pulse wheels 22 and 24, respectively. As the crankshaft (not shown) and camshaft (also not shown) rotate, wheels 22, 24 rotate along with them. The wheels 22, 24 possess teeth which can be sensed and measured by sensors according to measurement pulses generated by the sensors. The measurement pulses are detected by camshaft and crankshaft measurement pulse sensors 22a and 24a, respectively. The sensed pulses are used by a phase measurement device 26. A measurement phase difference is then determined. The phase difference is defined as the time from successive crank-to-cam pulses, divided by the time between successive crank pulses and multiplied by the angular distance corresponding to successive crank pulses (in degrees). In other words, the angular position difference is referenced to the difference between the cam shaft and the crank shaft. The measured phase difference may be expressed as θ0 16. This phase difference is then supplied to the control law 18 for reaching the desired spool position.
Referring to FIGS. 1 and 1A, a control law 18 of the closed-loop 10 is described in U.S. Pat. No. 5,184,578 and is hereby incorporate herein by reference. A simplified depiction of the control law is shown in FIG. 1A. Measured phase 26 is subjected to the control law 18 initially at block 30 wherein proportional-integral (PI) process occurs. Typically PI process is subdivided into two sub-processes. The first sub-process includes an amplification action; and the second sub-process includes an integration action. Measured VCT phase is further subjected to phase compensation at block 32
With regard to the phase compensator 32, due to phase measurement variations over the speed range of engines used for variable cam timing (VCT), it is desirable to have a method suitable for automatically adjusting the phase measurement variation. Further, because of the variable reluctance sensors used for sensing, it is necessary to implement in the method a way for compensating undesirable erroneous phase shift. Based upon testing, it has shown that while engine speed ranging from 500 to 6000 rpm, the phase may shift ranging from as much as 8° to as little as 1° with reference to crank shaft position. In addition, the amount of the phase shift varies from phaser to phaser so a fixed table in the method will only average the error. Therefore, it is desirous to have a method to automatically implement phase compensation at various engine speeds such as a set of predetermined values for correction being stored for use in an engine control unit (ECU).